Field of the Invention
The present invention relates to an organic light-emitting diode (OLED) display including a multi-mode cavity structure and having an improved light efficiency and color gamut. Furthermore, the present invention relates to an OLED display having an improved aperture ratio by forming a storage capacitor using a transparent conductive material.
Discussion of the Related Art
Recently, a variety of types of flat panel displays capable of reducing weight and volume (that is, the disadvantages of a cathode ray tube) are being developed. Such flat panel displays include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an electroluminescence (EL) device.
The EL device is basically divided into an inorganic EL device and an OLED device and is a self-emissive device. The EL device has advantages of high response speed, great emission efficiency and brightness, and a wide viewing angle.
FIG. 1 is a diagram showing the structure of an OLED according to the related art. The OLED includes an organic electroluminescence compound layer configured to perform electroluminescence and a cathode electrode and anode electrode configured to face each other with the organic electroluminescence compound layer interposed therebetween, as shown in FIG. 1. The organic electroluminescence compound layer includes a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), and an electron injection layer (EIL).
In such an OLED, excitons are formed in an excitation process in which holes and electrons injected into the anode electrode and the cathode electrode are recombined in the EML, and the OLED emits light due to energy from the excitons. An OLED display displays an image by electrically controlling the amount of light generated from the EML of an OLED, such as that of FIG. 1.
An organic light-emitting diode display (OLEDD) using the characteristics of an OLED, that is, an electroluminescence device, is basically divided into a passive matrix type organic light-emitting diode (PMOLED) display and an active matrix type organic light-emitting diode (AMOLED) display.
The AMOLED display displays an image by controlling an electric current flowing into an OLED using a thin film transistor (hereinafter referred to as a “TFT”).
FIG. 2 is an example of an equivalent circuit diagram showing the structure of a single pixel in an OLED display according to the related art. FIG. 3 is a plan view showing the structure of a single pixel in the OLED display according to the related art. FIG. 4 is a cross-sectional view taken along line I-I′ of FIG. 3 and shows the structure of an OLED display according to the related art.
Referring to FIGS. 2 and 3, an AMOLED display includes a switching TFT ST, a driving TFT DT connected to the switching TFT ST, and an OLED configured to come in contact with the driving TFT DT.
The switching TFT ST is formed at a portion where a scan line SL and a data line DL are intersected. The switching TFT ST functions to select a pixel. The switching TFT ST includes a gate electrode SG branched from the scan line SL, a semiconductor layer SA, a source electrode SS, and a drain electrode SD. Furthermore, the driving TFT DT functions to drive the OLED of a pixel selected by the switching TFT ST. The driving TFT DT includes a gate electrode DG connected to the drain electrode SD of the switching TFT ST, a semiconductor layer DA, a source electrode DS connected to a driving current line VDD, and a drain electrode DD. The drain electrode DD of the driving TFT DT is connected to the anode electrode ANO of the OLED.
More specifically, referring to FIG. 4, the gate electrodes SG and DG of the switching TFT ST and the driving TFT DT are formed on the substrate SUB of the AMOLED display. Furthermore, a gate insulating layer GI is covered on the gate electrodes SG and DG. The semiconductor layers SA and DA are formed on part of the gate insulating layer GI overlapping the gate electrodes SG and DG. The source electrode SS, DS and the drain electrode SD, DD are formed to face each other at a specific interval on the semiconductor layer SA, DA. The drain electrode SD of the switching TFT ST comes in contact with the gate electrode DG of the driving TFT DT through a contact hole formed in the gate insulating layer GI. A passivation layer PAS configured to cover the switching TFT ST and the driving TFT DT having such a structure is coated on the entire surface.
In particular, if the semiconductor layers SA and DA are made of an oxide semiconductor material, there are advantages in terms of high resolution and high-speed driving in a large-area TFT substrate having a high charging capacity attributable to a high charge mobility characteristic. However, the oxide semiconductor material may further include etch stoppers SE and DE for protecting a top surface from an etchant in order to secure the stability of the device. More specifically, the etch stopper SE, DE is formed to protect the semiconductor layer SA, DA from being etched back by an etchant which comes in contact with a top surface in a portion between the source electrode SS, DS and the drain electrode SD, DD.
A color filter CF is formed in a portion corresponding to the area of the anode electrode ANO to be formed later. The color filter CF may be formed to occupy a wide area, if possible. For example, the color filter CF may be formed to overlap a wide area of the data line DL, the driving current line VDD, and the scan line SL at the front. The substrate in which the color filter CF has been formed as described above does not have a flat surface due to several elements formed therein and has many steps. Accordingly, in order to make flat a surface of the substrate, an overcoat layer OC is coated on the entire surface of the substrate SUB.
Furthermore, the anode electrode ANO of the OLED is formed on the overcoat layer OC. In this case, the anode electrode ANO is connected to the drain electrode DD of the driving TFT DT through a contact hole formed in the overcoat layer OC and the passivation layer PAS.
A bank pattern BN is formed on an area in which the switching TFT ST, the driving TFT DT, and various lines DL, SL, and VDD have been formed in order to define a pixel area over the substrate in which the anode electrode ANO has been formed.
The anode electrode ANO exposed by the bank pattern BN becomes an emission area. An organic light-emitting layer OLE and a cathode electrode layer CAT are sequentially stacked on the anode electrode ANO exposed by the bank pattern BN. If the organic light-emitting layer OLE is made of an organic material that emits white light, it emits light of a color designated to each pixel by the underlying color filter CF. The OLED display having a structure of FIG. 4 is a bottom emission type display device which emits light downwardly.
In such a bottom emission type OLED display, a storage capacitor STG is formed in the space in which the gate electrode DG of the driving TFT DT overlaps the anode electrode ANO. The OLED display displays image information by driving the OLED. Very high energy is required to drive the OLED. Accordingly, a high-capacity storage capacitor is necessary to accurately display image information whose data value is rapidly changed, such as a moving image.
In order to sufficiently secure the size of the storage capacitor, the area of a storage capacitor electrode needs to be sufficiently large. In a bottom emission type OLED display, if the area of the storage capacitor is increased, there is a problem in that an area that emits light, that is, an aperture ratio, is reduced. In a top emission type OLED display, an aperture ratio is not reduced although a high-capacity storage capacitor is designed because the storage capacitor is able to be installed under the emission area. In the bottom emission type OLED display, however, there is a problem in that the area of the storage capacitor is directly related to a reduction of the aperture ratio.
Furthermore, recently, in order to improve light efficiency of an OLED display, an OLED display having a micro-cavity structure is being developed. In such an OLED display having a micro-cavity structure, light efficiency is greatly increased by a resonant effect between electrodes. However, the OLED display having a micro-cavity structure is problematic in that the color viewing angle is reduced because a spectrum bandwidth is very narrow. Accordingly, active research is recently carried out on an OLED display which prevents a reduction of the color viewing angle while improving light efficiency.